Spinal cord stimulation (SCS) using electrical pulses of constant or varying frequency, amplitude and pulse width has been done for many years to treat chronic neuropathic pain of the trunk and limbs. Usually after a percutaneous trial has shown efficacy, a complete medical device is implanted surgically, so that long-term therapy can be done, often for many years. The typical device has a pulse generator in a subcutaneous position that generates the electrical pulses, a multiwire extension to bring those pulses near to the spinal column, and a delicate lead or two with multiple electrodes to deliver the pulses within the spinal canal.
Early attempts placed the multielectrode leads next to the spinal cord so that neurons less than a millimeter away could be activated. This required significant and invasive surgery to open the dura covering the spinal cord, or to develop a space between the dura and the arachnoid membrane. It the lead moved or developed open circuits, efficacy was lost, or morbidity such as infection developed, the lead had to be removed or replaced with additional neurosurgical procedures. Such early leads were also delicate, to not compress the spinal cord.
Today, the preferred lead location is outside the dura, in the epidural space. This location has benefits such as quicker and easier surgical access to implant, minimization of unwanted side effects such as possible leakage of cerebrospinal fluid, and less difficulty to remove or replace the lead, should infection, loss of effectiveness, or lead migration or breakage occur. In this case, the electrodes are usually two to six millimeters away from the targeted neurons. Between them and the neurons to be excited are the dura, arachnoid membrane and a layer of highly conductive cerebrospinal fluid. These elements tend to diffuse the electrical currents, and boost the amplitudes needed for activation as much as ten-fold.
To better select the neurons that might be excited, multielectrode leads have been developed. The complete system allows programming, so that each pulse sent from the pulse generator can be delivered to the tissue through one or more cathodes and the current returns to the pulse generator through one or more anodes. Usually the neurons near the cathodes are depolarized sufficiently to create action potentials, especially at narrow pulse widths of 500 microseconds or less, when approximately square-wave pulses are used. It has been learned clinically and with the use of electrical models of the spinal cord (see Holsheimer J and Wesselink W A, Neurosurgery, vol. 41, pp 654–659, 1997) that the orientation of the anodes and cathodes with respect to the neurons is relevant. Activation usually requires that there be a component of the electric fields produced (actually, the second spatial derivative) that is parallel to the neuron's axon, and this can lead to electrical currents of sufficient intensity to initiate action potentials along axons.
There is a need to effectively stimulate two different vertebral levels to treat pain in different anatomical locations. It is also desirable to have the capability to steer the fields at these locations. Field steering may be provided by tripole stimulation. Tripole stimulation occurs when there is a set of three or more electrodes and at least two of the electrodes are pulsed overlapping in time. Tripole stimulation may be either transverse tripole stimulation (TTS) or longitudinal tripole stimulation (LTS). TTS is defined in this application as occuring when the first and second electrodes are positioned on opposite sides of an imaginary longitudinal axis that passes through the center of the third electrode and parallel to the longitudinal axis of the lead. LTS occurs when the electrodes are substantially oriented along the longitudinal axis of the lead.
Peer-reviewed publications of results from studies using devices delivering transverse tripole stimulation (TTS) have shown that TTS is quite effective in delivering paresthesia and relief of pain in the legs and feet when done at T10 to L1 vertebral levels (see Holsheimer J et al., Neurosurgery, Vol. 42, No. 3, pp 541–547, 1998; Wesselink W A et al., Neuromodulation, Vol. 2, No. 1, pp 5–14, 1999). However, when TTS was done at higher levels of T8–T9, specifically to treat low back pain, and even as low as L1, it was not shown to significantly relieve low back pain (see Slavin K V et al., Stereotactic & Functional Neurosurgery, Vol. 73, pp. 126–130, 1999).
Because TTS provides better results at certain anatomical regions and LTS to other anatomical regions it is desirable to have a lead having the capabilities to deliver TTS and LTS to the desired locations. Many of the anatomical regions for which TTS works well are at a lower vertebral level than the regions for which LTS works well. It is also generally preferred to perform orthograde insertion of leads (that is insertion in the direction from lower vertebral levels to higher vertebral levels). There is therefore a need to provide a method and lead that provides both TTS and LTS wherein the TTS electrodes are at a lower vertebral level than the LTS electrodes.